To improve driving safety of a vehicle, there has been proposed a vehicle lamp irradiating direction control system which executes swivel control to swivel the irradiating direction a headlamp so as to follow the steering direction. In such a system, in case of a system failure, the irradiating direction of the headlamp may be controlled to be directed in a direction different from the steering direction, whereby driving safety may deteriorate. Thus, to prevent such a problematic situation, there has been proposed a fail-safe technology for stopping the swivel control upon detection of a system failure. For example, a particular fail-safe technology fixes the irradiating direction of a headlamp in a straight-ahead direction of a vehicle when there is a system failure (see. e.g., Japan Patent Document No. JP 2005-132340 A). Further, a technique has been proposed for preventing the irradiating direction of the headlamp from being immediately directed in the steering direction when the system is restored after a failure. That is, when the system is restored during a rightward or leftward steering operation, the irradiating direction is abruptly changed from the straight-ahead direction to the rightward or leftward direction of the steering operation. This can impair driving safety because the illuminating region immediately changes from the region viewed by the driver to another region. Therefore, to prevent such situation, the foregoing system holds off restarting the swivel control until the steering angle is directed straight-ahead.
In the foregoing system, a target swivel angle is obtained by filtering a detected steering angle, and swivel control is executed based on the target swivel angle. For example, if the irradiating direction is controlled to follow the steering angle when there is a small fluctuation of the steering angle because of a rough road surface while the vehicle is running at high speed, the driver may become annoyed. As this is undesirable in view of driving safety, the steering angle is filtered to dampen an angular fluctuation to calculate the target swivel angle, based on which the irradiating direction is controlled, whereby a small fluctuation of the irradiating direction is prevented. As the result of this filtering, there exists an angular difference on the time axis between the actual steering angle and the target swivel angle. Therefore, there is a situation in which the steering angle is in the straight-ahead range but the target swivel angle is not in the straight-ahead range. If the system is restored and the swivel control is restarted in this situation, the irradiating direction of the lamp is controlled to be directed in the target swivel angle which is not yet within the straight-ahead range. Therefore, the foregoing system cannot fully address the problem described above in connection with the restoration of the system.
The problem which remains unsolved in the foregoing system is described in more detail with reference to FIG. 4A and FIG. 4B. For example, when the steering angle changes as shown in FIG. 4A, the steering angle is filtered to calculate the target swivel angle as shown in FIG. 4B. The swivel control normally is executed based on the target swivel angle. However, as shown in FIG. 4B, when there is a system failure, the irradiating direction is controlled to be in the straight-ahead direction irrespective of the target swivel angle. When the system failure is corrected, i.e. restored, the fail-safe operation is maintained until the steering angle is directed in the straight-ahead range. That is, when the steering angle is directed in the straight-ahead range, the fail-safe operation is deactivated, and the swivel control is restarted. At this time, however, the target swivel angle is not yet within the straight-ahead range because of the filtering. Therefore, the irradiating direction is controlled to jump from the straight-ahead direction to the target swivel angle. That is, the illuminating region immediately changes from the region viewed by the driver to another region, which can result in a feeling of discomfort for the driver and can deteriorate driving safety.